1. Field of the Invention
This invention relates to building construction apparatus and methods. In particular, this invention relates to apparatus for securing parts of a building to one another or to the ground for providing stability to the building against environmental conditions such as earthquakes, hurricanes and other forces seeking to lift, separate, or topple the building.
2. Description of the Prior Art
A common type of building structure comprises structural framing members to which shear diaphragms such as plywood sheets are secured to provide the frame with shear stability. Typically the framing members include upper and a lower horizontally extending members, sometimes referred to as the top plate(s) and the sill plate, respectively. These are separated from each other by vertically extending ribs, also known as studs. The horizontal and vertical members are typically nailed or screwed together to define a wall, as illustrated in FIG. 1. Adjacent walls are usually connected to each other with the aid of a second upper member secured to the first upper member in a staggered relationship to overlap the first at the joinder of the walls. This is often replicated through the various floor levels to the roof. The lower members, on the other hand, are secured to the floor of the building structure. The roof joists are, in turn, nailed or screwed to the second upper member.
Ground movement or high winds exert forces on the building that tend to lift the upper horizontal members relative to the ground or cause the vertical and horizontal members to rotate relative to each other. To reduce the effects of earthquakes, ground movement, and high winds such as hurricanes, structural members have been developed to provide down force stabilization. Hold-down mechanisms are designed to resist separation forces and provide uplift stabilization.
As mentioned above, a common stabilization technique is the nailing of plywood sheets to the structural framing members. The sheets are connected to adjacent sheets and the foundation by means of the structural framing members to which they are nailed. They thus form a unified structure that resists uplift and overturning forces. In order to further enhance the structural integrity of the structure, the sheets are secured to each other and to the foundation by means of hold-downs. U.S. Pat. No. 4,655,672 describes hold-downs in the form of brackets that are secured to framing members using bolts. The brackets are attached to the vertical framing members by means of bolts extending horizontally or to horizontal framing members by means of vertically extending bolts. The brackets are, in turn, connected to each other or the foundation. For example, in the case of vertically extending members, the brackets comprise L-shaped brackets in which one leg of the bracket extends outwardly from the framing member. The outwardly extending legs of adjacent pairs of brackets are connected to each other by means of threaded rods thus interconnecting the floors of the building. In this manner the brackets are connected to each thereby, in turn, connecting the framing members. The brackets are also connected to the foundation by means of threaded rods embedded in the foundation. A disadvantage of the above approach is the failure to accommodate wood shrinkage which causes the tension in the threaded rods to be lost, permitting a certain amount of play and possibly resulting in impact loads and shear wall nail failures that increase the risk of failure. Furthermore, a connection is required wherever a cross members is encountered. Thus a large number of parts is required. Also, a large number of holes have to be drilled and the brackets carefully aligned resulting in a time consuming and costly procedure. Due to the shape of the brackets, eccentric forces are exerted on the brackets, which are exacerbated by the small contact area of the bolts resulting in large pressure on the connections.
Earthquakes also create moving point forces which are directed upward and sideways through the foundation to the framing members and cause the shear diaphragms to rotate and buckle. To reduce rotational and point loading the movement of the upper corners of the shear diaphragms relative to their foundation has to be contained. A number of devices have been developed to achieve this. One solution proposed is described in U.S. Pat. No. 4,875,314 to Boilen in which tie rods extend from the foundation up through the various levels of the building and is secured at each level to an anchor by means of a nut and spread plate. The use of a rod extending upward from the foundation allows uplift forces to be transmitted directly to the foundation. However this system has severe limitation in that it fails to take account of wood shrinkage. As framing members shrink the initial snug connection between the tie rods and the anchors is lost. The framing members are thus no longer snugly restrained and expose the system to impact forces. Nails holding the framing members together are thus more likely to be pulled out, thereby further comprising the structural integrity of the building.
This limitation was partially addressed in U.S. Pat. No. 2,011,312 to Engman in which a connecting device is described that includes springs for taking account of dimensional changes of the structural elements. This device is, however, limited in its ability to hold the structural elements together. Specifically, the strength of the system is limited by the compressive force of the springs.
The present applicant addressed this issue in this earlier patent application, U.S. Pat. No. 5,535,561, by developing a hold-down device comprising a cable that is anchored at one end and is releasably clamped at the other end. A spring acts on the cable to take account of shrinkage by automatically moving the cable relative to the releasable clamp as the structural members shrink. A drawback of the system described in U.S. Pat. No. 5,535,561, however, is the cost of providing appropriate springs having the desired length and compressive force characteristics. This requires a large number of different springs to be made to accommodate the various applications.